1. Field of the Invention
This invention relates to the field of pressure sensors and, more particularly, to thin-film pressure sensors adapted for low pressure sensing applications.
2. Description of the Related Art
Pressure sensors have wide applicability today from microphones, household electronics, and simple vacuum sensing applications to biomedical instrumentation, wind tunnel, and automotive performance applications. Many of these devices depend on integrated sensor technology that utilizes the piezoresistive effect. Simply put, the piezoresistive effect is one in which the resistance of an element changes as a result of length changes for that element generally as a result of stress. Resistance changes can behave anisotropically in directions parallel to and perpendicular to the direction of the stress.
It is well known in sensor technology to employ a Wheatstone bridge arrangement of piezoresistors to sense pressure and pressure changes. The arrangement commonly chosen involves four piezoresistors located at four distinct edges of a pressure sensing membrane that is supported on a substrate. Generally, the position chosen for each resistor is one where the stress is greatest when vertical pressure is applied to the membrane at its the center or uniformly across the membrane surface. In these designs, orientation of the piezoresistors is also important. Usually, two piezoresistors are oriented parallel to the direction of stress so that their resistance increases with increasing pressure and the other two piezoresistors are oriented perpendicular to the direction of stress so that their resistance decreases with increasing pressure. See, for example, C. Malhaire et al., “Design of a polysilicon-on-insulator pressure sensor with original polysilicon layout for harsh environment,” Thin Solid Films, No. 427, pp. 362–366 (2003) and U.S. Pat. No. 6,700,174 issued to Miu et al. on Mar. 2, 2004.
Most proposed designs known in the art include rigid substrates. This limits the applicability of such pressure sensors to relatively high pressure environments. Many of the rigid substrates commercially known utilize metallic membranes such as stainless steel. These membranes are easily damaged in severe natural or corrosive environments.